1. Field of the Invention
The present invention relates to a die spring apparatus charged with compressed gas and adapted to be used in press equipment and the like.
2. Description of the Related Art
In stamping or drawing a blank by means of press equipment, a pressure pad is used to prevent the blank from wrinkling. The pad is urged by means of a die spring. The inventors hereof have been developing die spring apparatuses of a gas-spring type to be used in place of the conventional coil springs or urethane springs.
A conventional die spring apparatus 100 shown in FIG. 12 comprises a gas chamber 102 defined in a cylinder 101 and a rod 103 inserted in the cylinder 101. A proximal portion 105 of the cylinder 101 is supported by means of a first bearing portion 106. An outer end 107 of the rod 103 is in contact with a second bearing portion 108. The pressure of a compressed gas sealed in the gas chamber 102 acts in a direction to force out the rod 103 from the cylinder 101.
A first bearing 110 is provided at an open end of the cylinder 101. The bearing 110 is in contact with the outer peripheral surface of the rod 103. A piston 111 with a second bearing 112 thereon is disposed at the inner end of the rod 103. The bearing 112 is in contact with the inner peripheral surface of the cylinder 101. The bearings 110 and 112 allow the rod 103 to move in the axial direction, but prevent it from moving in the radial direction. Thus, the angle at which the rod 103 can swing with respect to the axis X0 of the cylinder 101 is about 0.015.degree. at the most.
A study made by the inventors hereof indicated that the die spring apparatus 100 constructed in this manner has the following problems.
Depending on the mounting state of the die spring apparatus 100, the axis X0 of the apparatus 100 sometimes may be tilted at an angle 81 to the bearing portion 108. In FIG. 12, the angle 81 is exaggerated for ease of illustration. If the apparatus 100 is tilted in this manner, a point 115 of contact between the rod 108 and the bearing portion 108 on the movable side shifts sideways for a distance .DELTA.P when the bearing portion 108 lowers for a stroke S with respect to the bearing portion 106 on the fixed side at the time of pressing operation.
Thus, a frictional force is produced between the rod 103 and the bearing portion 108, and a load F acts in the radial direction of the rod 103. In this case, a radial load R1 is produced in the first bearing 110, due to a moment corresponding to a distance L1 from the second bearing 112. If the distance from the first bearing 110 to the outer end 107 of the rod 103 is L2, R1 is given by EQU R1={(L1+L2).multidot.F)/L1.
Also, a radial load R2 is produced in the second bearing 112, due to a moment corresponding to the distance L1 from the first bearing 110. R2 is given by EQU R2=(L2.multidot.F)/L1.
Accordingly, the bearings 110 and 112 must be strong enough to stand the loads R1 and R2. At the time of pressing operation, however, the die spring apparatus 100 is subjected to a very great axial load. If the load on the rod 103 is 1,000 kgf, and if the coefficient of friction between the contact point 115 of the rod 103 and the bearing portion 108 is 0.2, for example, the load F is as great as 200 kgf. Accordingly, the loads R1 and R2 are also great, so that the bearings 110 and 112 are liable to be damaged. Further, the contact portion between the contact point 115 of the rod 103 and the bearing portion 108 wears, resulting in lowering of the durability of the apparatus.